Since a secondary battery, such as lithium-ion secondary batteries, has a small size and exhibits a large capacity, the secondary battery has been used in a wide variety of fields like cellular phones and notebook-size personal computers. Performance of a lithium-ion secondary battery is dependent on materials for the positive electrode, negative electrode and electrolyte constituting the secondary battery. Even among the materials, the research and development of active-material ingredients included in electrodes have been carried out actively. At present, carbon-based materials, such as graphite, are available as a negative-electrode active-material ingredient having been used commonly. Since a carbon negative electrode using graphite, and the like, as a negative-electrode active material, undergoes an intercalation reaction, the cyclability and output excel. Note, however, that no considerable upgrading in capacity is not expected from the carbon negative electrode from now on. In the meanwhile, a request for further upgrading lithium-ion secondary batteries in capacity has been intensifying as the lithium-ion secondary batteries' specifications and usages expand. Consequently, investigating negative-electrode active materials exhibiting a higher capacity, namely, a higher energy density, than carbon does, has been carried out.
As a negative-electrode active material being capable of materializing a high energy density, a silicon-based material, such as silicon or silicon oxide, is given. Silicon has a large lithium-ion sorbing (or occluding)/desorbing (or releasing) capacity per unit volume or per unit mass, and shows a high capacity being ten times or more of the capacity of carbon. However, although the silicon-based material has large charge and discharge capacities, the silicon-based material has a problem with the poor charge/discharge cyclability because of the following causes: the volumetric expansions at the time of sorbing lithium resulting in destructing electrodes; the pulverization of silicon leading to the downward sliding of the silicon off from electrodes; or the cut-off conductive paths resulting from the former two causes; and so on.
As a countermeasure for improving the charge/discharge cyclability of silicon, using a silicon oxide as a negative-electrode active material has been known. When a silicon oxide (e.g., SiOx where “x” falls in a range of 0.5≤“x”≤1.5) is heat treated, the silicon oxide has been known to decompose into Si and SiO2. The decomposition is called a disproportionation reaction. When the silicon oxide is SiO, homogenous solid silicon monoxide in which a ratio between Si and O is 1:1 roughly, internal reactions of the solid lead to separating the SiO into two phases, namely, an Si phase and an SiO2 phase. Of the two phases, the micro-fine Si phase mainly carries out sorbing and desorbing lithium. The SiO2 phase covers a plurality of the micro-fine Si phases, relieves the above-described volumetric expansion of the Si phases at the time of sorbing lithium, and furthermore possesses an action of inhibiting an electrolytic solution from decomposing. Therefore, a secondary battery, which uses a negative-electrode active material comprising SiO decomposed into an Si phase and an SiO2 phase, excels in cyclability. Moreover, the SiO2 phase has been known to form an Li—Si—O system compound at the initial charge to function as a lithium-ion conductor. However, since completely desorbing lithium sorbed in the Li—Si—O system compound at the time of discharging is difficult, the secondary battery is associated with such a problem that an irreversible capacity has arisen. In addition, in order for the Si phases inside an active material to sorb lithium therein and desorb lithium therefrom, the lithium is needed to diffuse within the SiO2 phase covering the Si phases to pass through the SiO2 phase. Therefore, another problem that the electric conductive property of the Li—Si—O system compound included in the SiO2 phase rate-determines the diffusion rate of lithium in the interior of an active material arises.
Patent Application Publication No. 1 discloses negative-electrode active-material particles for lithium secondary battery, a negative-electrode active-material particles comprising a first phase composed of an alloy between silicon and a metallic element, and a second phase in which silicon makes the major element. The second phase is involved in the first phase having a three-dimensionally networked structure. Since the first phase composed of the alloy has a higher electron conductivity than the second phase does, the current-collecting property of the active-material particles is enhanced.